When casting iron alloys, a modification of the iron can take place prior to casting by adding different alloying materials to the pouring ladle or to a special treatment ladle. A different manner is to supply alloying materials successively during the actual casting process. One example is the Inmold process. In the Inmold process which is used for manufacturing nodular iron alloys a reaction chamber is formed in the mould drag. At one edge the reaction chamber is connected to the sprue of the gating system via a short duct and at the other edge to a duct leading to the inlets to the casting. A certain amount of crushed FeSiMg alloy containing about 5% magnesium is placed in the reaction chamber. When casting, the iron flows into the chamber, the FeSiMg alloy melting on the surface and being gradually dissolved in the iron flowing through the reaction chamber. About 0.35% magnesium is dissolved in the iron which gradually fills the casting cavity. In the solidification, carbon is separated in the form of graphite as nodules, which characterises nodular iron. If the amount of magnesium is too low, the iron can wholly or partly solidify as grey cast iron, which has significantly lower strength. To prevent this, the reaction chamber is somewhat oversized. What is essential in the manufacture of nodular iron is that the amount of magnesium is not allowed to be lower than a certain minimum level. Higher contents than the standard value-do not produce any considerable detrimental effects.
The sectional area of the reaction chamber is decisive of the amount of magnesium that is dissolved in the iron at a given teeming rate (kg/s). The sectional area is dimensioned to an average teeming rate and is constant along the height of the reaction chamber. If the teeming rate is not constant during the casting process but decreases, this results in the magnesium content of the iron gradually increasing in inverse proportion to the teeming rate. This takes place, for instance, if the delivery head in casting decreases by part of the casting cavity being positioned above the parting line of the mould. When manufacturing nodular iron this does not cause any major problems as mentioned above, since it is possible to operate with safety margins for the addition of magnesium.
However, problems arise if compacted graphite iron is to be manufactured by the Inmold process. Compacted graphite iron is characterised in that the carbon dissolved in the iron is separated as vermiform graphite particles, not as spheres as in nodular iron, or as thin flaky structures as in grey cast iron. The compact graphite form is an intermediate form which only arises within a very narrow magnesium range which is dependent on, inter alia, the material thickness. A typical range is 0.01 to 0.013%. Using the conventional Inmold process where the sectional area of the reaction chamber is constant, the magnesium content can increase from 0.01 up to 0.02% if the teeming rate during the later part of the casting is reduced to half the initial rate. As a result, the iron having the higher magnesium content will contain a small amount of compacted graphite and a large amount of nodular graphite, i.e. a mixture of compacted graphite iron and nodular iron.
Another problem in the manufacturing of compacted graphite iron is that the lower limit of magnesium is dependent on the nucleation state of the base iron. The nucleation state can be measured indirectly using different methods, for instance thermal analysis, and for optimal conditions, it would be necessary to vary the percentage of magnesium in the iron in relation to the nucleation state. This is not possible with the traditional Inmold process.
One more problem of the traditional Inmold process is that part of the first iron that reaches the reaction chamber owing to the kinetic energy passes into the duct from the reaction chamber without having been in immediate contact with the alloying material. The reaction chamber is not completely filled with metal until after a few seconds. This means that the first iron which flows into the casting cavity may in some cases have too low an alloying material content.